This poster includes instructions that you should not burn zombies as this will release airborne toxins which will spread the infection. I have never found this to be the case. Perhaps these instructions pertain to a zombie infectious agent of which I am not familiar.
In place of burning/incineration, this poster seems to recommend an acid bath for disposal. I suppose that will work too. A shallow grave and copious amounts of quicklime may also do the trick. But maybe this unknown infectious agent will contaminate the soil. I don't know.
I say deal with the Walkers you got as best you can and try to limit the spread of infection with the tools you have available.